1. Field of the Invention
The present invention relates to an amplitude servo pattern, a magnetic recording medium and the manufacturing method. More specifically, the present invention is directed to an amplitude servo pattern having amplitude reproducing servo signals, a magnetic recording medium having thereon a magnetized bit pattern corresponding to the amplitude servo pattern, and a manufacturing method for manufacturing the magnetic recording medium.
The present invention also relates to a patterned magnetic transfer master substrate for use in the manufacturing method for manufacturing the magnetic recording medium.
Further, the present invention is directed to a magnetic recording/reproducing apparatus that uses the magnetic recording medium having thereon the magnetized bit pattern corresponding to the amplitude reproducing servo signals.
2. Description of the Related Art
Generally, in the field of magnetic recording media, inexpensive media with a larger capacity for storing information have been preferred, with increasing amount of information. At the same time, the media are preferable to allow the so-called fast access for reading out required data in a short time. Consequently, various types of high density magnetic recording media are known. The high density magnetic recording media have the information recording region formed of narrow tracks. In order for the magnetic head to accurately scan over the narrow tracks to reproduce a signal of high S/N (signal-to-noise) ratio, the so-called tracking servo technology plays an important role. For the tracking servo, the sector servo technology is widely used.
The sector servo technology employs servo fields arranged regularly at predetermined angles on the data surface of a magnetic recording medium, such as a magnetic disk or the like. The servo field contains servo information including a servo signal for tracking, track address signal, reproducing clock signal, and the like, and the reproducing magnetic head scans over the servo field to read out the servo information for confirming or correcting its position.
Generally, reproduced amplitude information of the servo signal is used as the tracking servo signal. In a general servo pattern, the servo signal comprises A, B, C and D bursts, and respective bits in the A and B burst bit strings forming A and B bursts respectively are arranged away from each other by ½ track width from the center of the track. When the reproducing magnetic head passes over the servo fields, tracking servo is applied to the head such that the amplitude reproduced by A and B burst bit strings becomes equal to each other.
The servo information needs to be prerecorded on a magnetic recording medium as a preformat when the medium is manufactured. Currently, the preformatting is implemented using a dedicated servo recording device. The servo recording device currently used has a magnetic head with the width, for example, of 75% of the track pitch. Servo signals are recorded by rotating the disk with the head placing close to the disk, and moving the head by ½ track from the outer to inner circumferences. Consequently, the preformatting of a single disk requires a long time, posing a problem from the stand point of production efficiency.
In the mean time, a method for transferring a pattern carrying servo information formed on a master substrate to a magnetic recording medium through magnetic transfer is proposed as described, for example, in Japanese Unexamined Patent Publication Nos. 10 (1998)-040544 and 10(1998)-269566.
The method uses a patterned master substrate having a transfer pattern, which comprises an irregular pattern corresponding to the information to be transferred to a magnetic recording medium (slave medium), such as a magnetic disk or the like. In transferring the information from the master substrate to a magnetic recording medium, the substrate and the medium are placed together in close contact with each other and a transferring magnetic field is applied, thereby a magnetized pattern corresponding to the irregular pattern of the master substrate is magnetically transferred to the magnetic recording medium. The method affords the advantage that it allows the static information transfer without changing the relative positions between the master substrate and magnetic recording medium, that is, it allows accurate preformatting of the magnetic recording medium within an extremely shorter time.
The amplitude servo pattern described above has a servo burst signal composed of burst bit strings disposed on different tracks and arranged adjacently in a track width direction with spacing of approximately 1 track pitch as illustrated in Japanese Unexamined Patent Publication Nos. 10(1998)-040544 and 10(1998)-269566. The adjacently disposed burst bit strings are formed to have the same phase, and a non-signal region of approximately 1 track pitch is present between them. When such a servo pattern is recorded on a magnetic recording medium through magnetic transfer, magnetization that causes a noise problem occurs in the non-signal region.
The phenomenon described above will be described with reference to FIG. 5. The servo burst signal shown in the drawing is a partial enlarged view of a pattern formed on a patterned magnetic transfer master substrate. The servo burst signals comprise an irregular transfer pattern of a magnetic material, which is placed together with a magnetic recording medium in close contact, and a transferring magnetic field is applied so that a magnetized pattern that corresponds to the irregular pattern is recorded on the magnetic medium. A burst bit string arranged on the first track has a plurality of burst bit elements. But, here in FIG. 5, only two elements 51a, 51a are shown for clarity, which are disposed in the track direction x with spacing corresponding to the width of the element in the track direction. On the second track, two burst bit elements 52a, 52a are adjacently disposed in the track width direction r such that they have the same phase as the elements 51a, 51a respectively with a spacing of approximately one track (hereinafter, this spacing is referred to as the “intermediate track” for convenience). When an external magnetic field for the magnetic transfer is applied, the edges of each of the burst bit elements 51a, 51a, 52a and 52a are positively and negatively polarized respectively as shown in FIG. 5.
Thus, magnetic fields having the same phase are present on the intermediate track. That is, those shown by arc shaped arrows 51p erupted from the burst bit elements 51a, 51a on the first track, and those shown by arc shaped arrows 52p erupted from the burst bit elements 52a, 52a are present on the second track. Consequently, the magnetic field intensity range in the width direction of the intermediate track indicated by a double head arrow W becomes like that as shown by the dashed line 5 in FIG. 6, which is directly transferred to the magnetic recording medium. The referent of the “magnetic field intensity range” as used herein means the magnitude of the change in the magnetic field intensity in a track direction at a particular point in the width of the intermediate track between the two adjacent burst signals in a track width direction (for example, the width indicated by the double head arrow W in FIG. 5). Accordingly, when the intermediate track between the first burst bit string and second burst bit string recorded on the magnetic recording medium is scanned by the magnetic head, the magnetic field indicated by the dashed line 5 in FIG. 6 is detected as the noise, causing degraded capability of the magnetic head for accurate tracking.
Today, in particular, in the trend toward an increasingly large capacity for magnetic disk media, efforts have been made for a narrower track width, and a magnetic disk medium with a track pitch of, for example, around or less than 200 nm is within the reach of reality. A narrower track pitch means smaller amplitude reproduced by each of the burst bit strings, and the tracking accuracy may be affected significantly by the noise described above.